This invention relates generally to cathode ray tube (CRT) monitors and specifically to monitors that have different operating modes in which the vertical resolution is changed.
Monitors embody CRT display devices and horizontal and vertical deflection systems for scanning the electron beam in the CRT across its phosphor target or faceplate to produce a raster. They also include video processing means for processing input video information for display on the CRT. With alpha-numeric displays, a higher degree of resolution is desirable. In response to this need, monitors having the capability of more than one vertical resolution, as well as different horizontal scan rates, are now available. Such monitors must be able to respond to the particular type of display for which the input video information is formatted. The display or format is encoded by the polarities of the incoming vertical and horizontal sync signal information that is supplied with the video data. The monitor must be able to decode that information to establish the correct operating mode and to develop horizontal and vertical sync pulses of correct polarity for the deflection circuits in the monitor.
The present invention is directed to a simple, low cost automatic vertical size control mechanism. The different vertical modes are defined by vertical resolutions of 400 lines and 480 lines. The 400 line resolution screen is obtained ty increasing the vertical size or sweep voltage (effectively overscanning the CRT) and only displaying 400 lines on the CRT.